1. Field of the Invention
This invention relates to the processing of optical beams, and more particularly to optical notch filters used to discriminate against optical radiation at selected frequencies.
2. Description of the Related Art
Optical beams may be presented as a combination of coherent and noncoherent radiation, and it is sometimes desirable to remove the coherent components. When a light wave exhibits complete coherence, there is a predictable connection or correlation between the amplitude and phase at any one point in the beam and at any other point. In general, a cross-section of the beam will reveal a wave front, every portion of which is in-phase with every other portion. With noncoherent radiation, on the other hand, the frequency, phase and amplitude at any point in the beam is random with respect to other points.
Notch filters have been used in a variety of applications to remove signal components with a given frequency, or falling within a relatively narrow frequency band, from an overall signal. Notch filters have been developed for optical radiation uses, and have been employed to remove coherent radiation from a beam. However, presently available optical notch filters are passive devices in the sense that, while they may be tuned to different frequencies, once they are set their tuning remains fixed and they do not respond to dynamics within the beam. Thus, the filters will not respond if they are originally set to the frequency of the coherent radiation, but the frequency of the coherent radiation within the beam thereafter changes. Also, the coherent radiation frequency may be unknown, and the presence or absence of coherent radiation may itself also be unknown. In these situations present filters are ineffective.